1. Field of the Invention
This invention relates to a method and an apparatus for forming a plate-like work to a shape with a double-curved surface like ship's shell plates which bear a complicated three-dimensionally curved surface.
2. Description of the Prior Art
The plates with a double-curved surface, which are extensively used especially as shell plates of ships, have thus far been formed by a combination of partial pressing operation using approximate dies of the desired shape, and linear heating operation using a burner for thermoplastically bending the plates to the aimed shape with reference to a gage. However, this method necessitates, in addition to an extremely large number of dies of different shapes, manual operations for the linear heating, which require not only a great deal of time but also skilled operators with a high degree of experience and discernment.
Recently, for automating the operation of forming shell plates of ships, a research group of The Japan Shipbuilding Society proposed a universal press with multiple piston heads, having a large number of punches arrayed in the fashion of a grid on the upper and lower sides of a steel plate to be formed, varying the heights of the punches by numeric control so that a group of punches approximate a continuous die for forming a double-curved surface. However, according to this method, the number of the punches has to be increased to some extent in order to achieve satisfactory working accuracy, resulting in production facilities of unduly large size and involving an extremely large number of control points.
Further, Technical Report Vol. 13, No. 6 (1976) of Mitsubishi Heavy Industries, Ltd. proposes a triple-raw-press for forming a double-curved surface, realizing a reduction of the number of control punches to 30 units, which is a remarkable reduction as compared with the afore-mentioned universal press with multiple piston heads but still requires a large number of punches.
Although the punch heads are mounted on spherical seats to permit head tuning motions relative to the respective punch rods in both of the afore-mentioned universal press and triple-row-press, the punch heads are necessarily deviated in tilted directions from the respective centers during the head turning motions. The paired punches which are located on the upper and lower sides of a work are deviated in the opposite directions since the direction of curvature on one side of the work is reversed as compared with the direction on the other side of the work. Consequently, the upper and lower punch heads are caused to contact and press the work at different points, resulting in a deteriorated double-curved surface formability.